Continue to take sildenafil even if you feel well. Effects of VIAGRA on Vision: At single oral doses of mg and mg, transient dose-related impairment of color discrimination was detected using the Farnsworth-Munsell hue test, with peak effects near the time of peak plasma levels. May make this condition worse. John’s wort. Sildenafil should not be used with any other medicine or device that causes erections. Mean sildenafil plasma concentrations measured after the administration of a single oral dose of mg to healthy male volunteers is depicted below: Figure 5: Mean Sildenafil Plasma Concentrations in Healthy Male Volunteers. If you experience a sudden loss of vision in one or both eyes, contact your doctor immediately.The average length of time Viagra lasts. On average, Viagra lasts between 2 and 3 hours before its effects start to diminish. Viagra can last up to 5 hours or longer depending on your dosage, your. AdSave Time & Money with Online Prescription for Viagra, Cialis, Sildenafil and Tadafil. Free & Private Online Consultant with Real Doctors. Reliable E.D. Meds Shipped DiscreetlyK+ visitors in the past monthFree Shipping · Real Doctors · Discreet Packaging · Personalized Plan. Examples of Viagra’s more commonly reported side effects include: headache. flushing (temporary warmth, redness, or deepening of skin color) stuffy nose. indigestion (upset stomach) mild vision.